This invention relates generally to the field of working metal by lathe turning. More particularly, the invention encompasses a method for cutting extremely small grooves in hard tool steel.
The requirement to cut small grooves in a turning blank of hard tool steel may arise in a variety of applications. One such application is in the manufacture of heat exchanger tubing. Either one or both of the surfaces of such tubing is frequently configured with, for example, a pattern of ribs or fins in order to provide an enhancement in heat transfer performance over tubing having smooth surfaces. One such type of an enhancement is a pattern of longitudinal ribs on the internal surface of the tubing. This type of enhancement readily adapts itself to use in a manufacturing process in which flat metal strip is roll formed into a cylindrical shape with the resultant longitudinal seam being welded to form the tubing. A surface enhancement embossing step can be added to the roll forming and welding process so that the pattern of longitudinal ribs can be formed on the flat strip before it is roll formed and welded into a finished tube. Modern equipment and techniques enable the manufacture of quantities of tubing at extremely high speeds by this process.
To form the ribs, the metal strip is pressed between two rollers. At least one of the rollers has a pattern of grooves that is the mirror image of the rib pattern to be embossed into the strip. Under the pressure exerted by the two rollers, strip metal, usually copper, flows into the grooves to form the ribs of the enhancement pattern. The width of the metal strip needed to form tubing of the size commonly used in refrigerant-to-air heat exchangers in air conditioning and refrigeration systems is about 3 cm. On a strip this width will be embossed a surface enhancement comprising somewhere in the range of 70 to 80 ribs, each rib having a height on the order of 0.25 mm. The ribs are thus relatively very small. It follows that the size of the grooves in the embossing roller is correspondingly very small.
To withstand wear, the embossing rollers must be made of a very hard material such as a tool steel. When in operation, there are not only compression forces acting on the grooved embossing wheel but also shear forces acting on the metal between adjacent grooves on the roller. For this reason, the metal of the roller must also be very tough and the configuration of the grooves must be such as to reduce the incidence of localized concentrations of stress in the metal between the grooves of the roller. There should be a high degree of uniformity in the planforms of the individual grooves in the grooved embossing roller.
Forming grooves of the desired size and shape in a metal hard enough and tough enough to be suitable for an embossing roller presents unique challenges. One method of forming grooves is by grinding them in to the roller with a suitable abrasive wheel. This is a time consuming and therefore expensive task. One reason for the time required is that the abrasive wheel must be relatively frequently dressed up. Otherwise, the uniformity of the grooves is degraded because of abrasive wheel wear. In addition, one is not able to achieve certain specific groove configurations because of limitations in the grinding art.
What is needed is a method of forming very small grooves in a very hard and very tough tool steel blank. The method should be capable of forming grooves of a variety of cross sections and not be excessively time consuming.